It is desirable that the exhaust noise of a vehicle is reduced from low rotation speed to high rotation speed of an engine without increasing exhaust pressure loss. WO95/13460 published by the World Intellectual Properties Organization (WIPO) in 1995 for example discloses a muffler wherein a valve responding to exhaust pressure is provided, and the flowpath of exhaust inside the muffler is changed according to the exhaust pressure.
In this device, first and second expansion chambers and a volume chamber are formed inside the muffler. When exhaust pressure is low, exhaust gas introduced into the muffler by an inlet tube is discharged from a tail tube through the two expansion chambers. When exhaust pressure exceeds a predetermined value, a part of the exhaust gas flows from the first expansion chamber into the second expansion chamber via the volume chamber. The above-mentioned valve is installed in a pipe connecting the volume chamber and the second expansion chamber.
In this case, considering the exhaust gas flowpath which does not pass through the volume chamber as a first path, and the exhaust flowpath which passes through the volume chamber as a second path, the second flowpath functions as a resonator having one degree of freedom with respect to the first flowpath.
The sound wave which passes through the first path is shifted by 180 degrees relative to the sound wave which passes through the second path with the resonance frequency as a boundary, so the pressure waves in the two paths effectively have reverse phase. Therefore the sound waves at the confluence of the two paths in the second expansion chamber mutually interfere which brings about the noise reduction effect.
However, as the resonator formed by the second path relative to the first path in this device effectively has one degree of freedom, in the frequency regions extending beyond anti-resonance frequency on both sides of the resonance frequency, the sound pressure level of the second path drops, and a large difference therefore arises from the level of the first path in these regions.
Therefore, although the sound waves traveling through the first and second paths have reversed phase in the confluence part, a noise reduction. effect due to mutual interference of sound waves can be expected only in a narrow region around the resonance frequency.
Further, if a plurality of expansion chambers are provided in the muffler, the volume of each chamber is small, and muffling performance at low frequencies is not very high.